The term "ink jet" as used herein is intended to include all drop on demand ink jet propulsion systems, including, but not limited to, "bubble jet," "thermal ink jet," and piezoelectric.
Drop on demand thermal ink jet printers operate by rapidly heating a small volume of ink, causing it to vaporize and expand, thereby ejecting ink through an orifice or nozzle and causing it to land on selected areas of a receiving medium. The sequenced operation of an array of such orifices moving past a receiver writes a dot pattern of ink on the receiver, forming text or pictorial images. The print head typically includes an ink reservoir and channels to replenish the ink to the region in which vaporization occurs. An arrangement of thermal ink jet heaters, ink channels, and nozzles is disclosed in U.S. Pat. No. 4,882,595. Also known is an ink jet printing device which electrically generates an agitated condition between an electrode and a counter electrode, which in turn causes ink particles to be emitted through the nozzle. (U.S. Pat. No. 4,432,003). Another class of devices use a separate piezoelectric transducer to expel the drops. Color rendition is accomplished by adding a few (typically three) color ink reservoirs and associated nozzles and ejection means so that dots of different colors may be overlaid on an appropriate receiver.
Although the drop on demand printers are efficient and inexpensive, the images they produce are in general binary in the sense that the size of the drops of ink cannot be much varied and the number of colors available for each drop is small, being that of the number of associated ink reservoirs and nozzle sets. While European Pat. No 0 468 075 teaches the use of multiple resistive heater elements with voltage pulses tailored to control droplet volume, the variation in volume is not optimally large. Also, variation of the area of the dots on the receiving medium, which results from droplet volume variation, is not an optimal method for producing a continuous tone image, compared with variation of color intensity within dots of constant area.
While multilevel black and white or multilevel color dots can be achieved by multiply depositing a variable number of identical drops of ink in the same spatial location, this greatly slows the operation of the printer because the frequency of operation of the droplet ejection process is limited. For example, U.S. Pat. No. 4,631,548 teaches a method of multiple droplet deposition in which the diameter of the matrix dot formed on the recording media is held nearly constant. Similarly, halftoning may be practiced, as is well known in the printing industry, but the required number of nozzles is then very large and/or the printing speed is again substantially reduced.
It is thus desirable to control the intensity of the color droplets or of the black ink droplets produced in order to render superior image quality while maintaining machine productivity. Some techniques to accomplish this objective have been previously disclosed. U.S. Pat. No. 5,221,934 teaches a method for electrochemical resistive ink jet printing comprising a solvent and a leuco dye in which the passage of a variable current through a leuco dye produces an ink of variable density. Also U.S. Pat. No. 4,503,444 teaches an operational mode of thermal ink jet printing in which the amount of ink in a droplet may be controlled by formation of the droplet from the coalescence of many smaller droplets emitted at very high repetition rates. These methods require either specialized inks or specialized operating conditions, and may produce dots of varying sizes rather than the more desirable case of dots of constant size but varying color intensity.